1. Field of the Invention
This invention relates to switched capacitor filters, and more specifically to switched capacitor filters useful in the generation of exponential envelope voltages used, for example, in electronic organs.
2. Description of the Prior Art
Prior art methods of generating exponential envelope voltages for use in electronic organs are well known. The circuit as shown in FIG. 1 is commonly used for this purpose (see, for example, the article by David L. Fried entitled "Analog Sample-Data Filters" published on pages 302 to 304 of the IEEE Journal of Solid-State Circuits in August 1972). In the circuit of FIG. 1, a reference voltage (V.sub.REF) is applied at node 23. This reference voltage may be obtained by any well known means, the simplest of which is a voltage divider consisting of resistors 31 and 32 connected between a source of potential at node 30 and ground as shown in FIG. 1. The two nonoverlapping clock signals required to operate the circuit of FIG. 1 are shown in FIG. 2, and are labeled .phi. and .phi.. The switches used in the circuit of FIG. 1 are shown to be MOSFET transistors 11 and 12; however, any suitable switch means may be used. In the operation of the circuit of FIG. 1, initially the voltages appearing on capacitors 13 and 14 are zero. During the first half clock period when .phi. is high, switch 11 is turned on and capacitor 13 (having a capacitance value C.sub.1) is charged through switch 11 to V.sub.ref as applied to node 23. The amount of charge stored on capacitor 13 is simply C.sub.1 .multidot.V.sub.REF. During the second half of the first clock period, .phi. is high, and .phi. is low. This causes switch 11 to turn off and switch 12 to turn on. Thus the charge previously stored on capacitor 13 is shared with capacitor 14 (having capacitance value C.sub.2) through the path provided by the open switch 12. The resulting voltage across capacitor 14 is thus Q/(C.sub.1 +C.sub.2) or [C.sub.1 /(C.sub.1 +C.sub.2)]V.sub.REF .multidot..phi. then goes low, and .phi. goes high. During the first half of the second clock period, capacitor 13 is again charged to V.sub.REF through switch 11. During the second half of the second clock period .phi. goes low and .phi. goes high thus causing the charge stored on capacitor 13 to again be shared with capacitor 14. Thus the voltage on capacitor 14 becomes ##EQU1## A graphical representation of the voltage available at node 22 with respect to time is shown in FIG. 3. Note that the initial step is rather large, and subsequent steps occurring during subsequent clock cycles become smaller and smaller, thus resulting in an approximately exponential voltage rise on node 22.
An RC circuit equivalent to the circuit of FIG. 1 is shown in FIG. 4. With a reference voltage applied to terminal 23, capacitor 14 will charge through resistor 45 resulting in a voltage varying exponentially with time appearing on terminal 22. The circuit of FIG. 1 will approximate this RC circuit with resistor equivalent capacitor 13. The time constant of the circuit of FIG. 1 will be equal to tC.sub.2 /C.sub.1 where t is the period of clock pulses .phi. and .phi.. Thus, the time constant of a switched capacitor equivalent circuit may be changed simply by changing the period of .phi. and .phi.. Furthermore, in MOS integrated circuits, resistance values are not highly controllable due to process limitations, while capacitance ratios are highly controllable, because capacitor size is quite controllable, and dielectric thickness is quite uniform across each semiconductor die. The high resistor values required to generate slow exponential voltages would also consume too much space on a semiconductor die to be practical. For these reasons, switched capacitor "resistor-equivalent" circuits are favored over simple RC circuits in MOS applications.
One disadvantage in the prior art circuit of FIG. 1 is due to the parasitic capacitance inherent in MOSFET transistors. Such a parasitic capacitor is shown in dashed lines in FIG. 1 as capacitor 15 appearing between gate 9 and drain 10 of MOSFET 12. This causes the output voltage stored on capacitor 14 and available on node 22 to be degraded due to charge sharing with capacitor 15. Thus, after the first clock period, when .phi. goes low and switch 12 turns off, the actual voltage available on capacitor 14 will be approximately equal to ##EQU2## where C.sub.3 is the capacitance value of parasitic capacitor 15. This voltage degradation is referred to as "pickoff". Pickoff generates a noise component seen across capacitor 14 and appearing on node 22. The frequency of pickoff is equal to the sampling frequency, f.sub..phi..